The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland
Introduction
Uranium series disequilibria in young oceanic basalts provide constraints on the dynamics of partial melting and melt extraction processes in the upper mantle. Specifically, they reveal information about mantle upwelling velocity, melt extraction rate, and residual porosity, but also about intrinsic properties of the mantle source such as its modal mineralogy and melt productivity (e.g., Condomines et al., 1981, Williams and Gill, 1989, McKenzie, 1985, Cohen and O’Nions, 1993, Iwamori, 1993, Spiegelman and Elliott, 1993, Turner et al., 1997, Bourdon et al., 1998, Bourdon et al., 2005, Bourdon et al., 2006, Sims et al., 1999, Stracke et al., 1999, Stracke et al., 2006, Stracke et al., 2003a, Peate et al., 2001, Kokfelt et al., 2003, Lundstrom et al., 2003, Pietruszka et al., 2009, Prytulak and Elliott, 2009).
During partial melting of the upper mantle, differences in residence time of the parent and daughter nuclides in the melt and residual solid mantle cause U-series disequilibrium. Proposed melting models range from ‘dynamic melting’ assuming rapid melt extraction with no equilibration between the partial melts and solid (McKenzie, 1985) to ‘equilibrium porous-flow’ with continuous melt–solid equilibration (Spiegelman and Elliott, 1993). More complex models suggest a so-called two-porosity regime during melt extraction, which implies different degrees of melt–solid equilibration at different depths in the mantle (Iwamori, 1994, Lundstrom et al., 2000, Lundstrom, 2001, Jull et al., 2002). These melting models can be used to explain the observed U-series isotope variation by variation in mantle upwelling velocity, which is directly proportional to the melting rate, and residual porosity during partial melting. Generally, the above-mentioned models make the simplifying assumption that the mantle source has a homogeneous mineralogical composition. The presence of lithological heterogeneity in the mantle, however, may change its melting behaviour, because different lithologies have different modal composition, trace element partitioning characteristics, and melt productivity (Lundstrom et al., 1995, Bourdon et al., 1996, Hirschmann and Stolper, 1996, Stracke et al., 1999, Stracke et al., 2003a, Pertermann and Hirschmann, 2003, Pertermann et al., 2004, Prytulak and Elliott, 2007, Prytulak and Elliott, 2009).
Melt productivity for example – defined as the amount of melt formed per increment of pressure release – is consistently larger for mafic lithologies compared to peridotites (Hirschmann and Stolper, 1996, Asimow et al., 1997, Asimow et al., 2001, Hirschmann et al., 1999a, Kogiso et al., 2004, Pertermann and Hirschmann, 2003) resulting in higher melting rates. Prytulak and Elliott (2009) pointed out that the differences in melt productivity and thus melting rate between peridotitic and pyroxenitic mantle components could have a much larger effect on the U-series nuclides in ocean island basalts (OIB) than differences in mantle upwelling velocity. Russo et al. (2009) suggested that melting of fertile pyroxenite veins could explain the relationships between trace element ratios and 230Th and 226Ra excesses observed in mid ocean ridge basalts (MORB) from the South-East Indian Ridge. The quantitative effect of lithological source heterogeneity on the U-series disequilibria, however, remains difficult to predict due to the uncertainty attached to the relevant melting and partitioning behaviour of lithologically different sources (e.g., Stracke et al., 1999, Bourdon and Sims, 2003).
In Icelandic rocks, correlations between major elements and trace element ratios and long-lived isotopes suggest that melting of at least two components, one isotopically depleted and one isotopically enriched, is required to explain the observed trends (Wood, 1981, Elliott et al., 1991, Maclennan et al., 2003, Stracke et al., 2003b, Kokfelt et al., 2006, Maclennan, 2008a, Stracke and Bourdon, 2009, Peate et al., 2010, Koornneef et al., 2012). Although the nature of the enriched Icelandic source component remains controversial, most previous studies favoured ancient recycled oceanic crust, present in form of small-scale mafic components (e.g., Chauvel and Hémond, 2000, Skovgaard et al., 2001, Stracke et al., 2003b, Kokfelt et al., 2006, Peate et al., 2010). Enrichments in Nb/La and Nb/U ratios combined with higher 206Pb/204Pb in samples from the Western Rift Zone and the Reykjanes Peninsula suggest that the abundance of this enriched component is larger beneath these main rift areas compared to the Northern Rift Zone (Hanan et al., 2000, Koornneef et al., 2012). The inferred lithological source heterogeneity may therefore affect the U-series disequilibria in young Icelandic lavas. Kokfelt et al., 2003, however, mainly attributed the observed differences in (230Th/238U) (where parentheses denote activity ratios) in Icelandic rift-zone lavas to variations in mantle upwelling rate as a result of decreasing mantle potential temperature away from the plume centre (Kokfelt et al., 2003, Bourdon et al., 2006).
Here, we present new 226Ra–230Th–238U and 231Pa–235U disequilibria data on 25 geochemically well characterised post-glacial tholeiites from Iceland’s main rift areas (Koornneef et al., 2012) and replicate analyses of four lavas from Theistareykir previously analysed by Stracke et al., 2006, Stracke et al., 2003a). In addition to 230Th–238U disequilibria, the aim is to use (231Pa/235U) ratios, which are more sensitive to variability in melting rates compared to (230Th/238U) ratios, to evaluate the potential effects of source heterogeneity and the inferred variations in regional upwelling velocity on the U-series disequilibria.
Even though variations in mantle potential temperature have no resolvable influence on the major, trace element, and long-lived isotope systematics (Koornneef et al., 2012), our (230Th/238U) data demonstrate that systematic variation in regional mantle upwelling velocity across Iceland is required (Kokfelt et al., 2006, Bourdon et al., 2006). Variability of the (230Th/238U) and (231Pa/235U) ratios on a local scale and the observed correlations with highly incompatible trace elements reveal an important role of source heterogeneity for establishing the U-series disequilibria in Icelandic rift zone lavas.
Section snippets
Sample preparation and analytical techniques
U, Th, Pa and Ra concentrations and isotope ratios were determined on 25 post glacial tholeiites from Iceland’s main rift areas (Fig. 1). Koornneef et al. (2012) previously reported their major and trace element and Hf and Nd isotope composition. In addition to the samples from the Reykjanes Peninsula (RP, n = 10), the Western Volcanic Zone (WV, n = 7), and the Northern Volcanic Zone (NV, n = 8), we re-analysed four samples from Theistareykir, a small area in the Northern Volcanic Zone, that were
Results
U–Th–Pa and Ra concentrations and 230Th–238U, 231Pa–235U and 226Ra–230Th disequilibria data are presented in Table 1 and Fig. 2, Fig. 3, Fig. 4. Data corrected for post-eruptive decay (Table 1), which is mainly relevant for the 226Ra-disequilibria of the lavas, are also shown in Fig. 3. Note that the accuracy of the age-corrected data is limited by the precision of the available age estimates for each lava flow (Peate et al., 2009, Sinton et al., 2005).
U-series melting models
Several U-series melting models have been proposed (e.g., McKenzie, 1985, Iwamori, 1993, Spiegelman and Elliott, 1993, Lundstrom et al., 1999, Jull et al., 2002). The difference in residence time between the parent and daughter nuclides causes in-growth of the daughter nuclide when the parent nuclide is retained preferentially in the solid residue during partial melting. Since the Iceland lavas show evidence for compositional variation created within melt channels (Maclennan et al., 2007,
The role of crustal processes
Secondary processes such as crystallisation of phases that fractionate the U-series isotopes, radioactive decay during magma storage, and assimilation of hydrothermally altered wall rocks or evolved lavas potentially disturb the melting-induced U-series disequilibria.
U, Th, Pa and Ra are all highly incompatible in olivine, clinopyroxene and plagioclase (Blundy and Wood, 2003, Fabbrizio et al., 2009), which crystallise in the tholeiitic basalts analysed here (Koornneef et al., 2012). Thus their
Conclusions
230Th excesses in recent Icelandic lavas correlate with distance from the plume centre and the mean variation in U-series disequilibria can be explained by mantle upwelling velocities of ∼14 cm/yr at the plume axis to ∼4 cm/yr at the plume periphery. Both the absolute value and the range of inferred upwelling velocities are, however, model-sensitive. The comparatively few (231Pa/235U) data reported here do not substantiate the inferences from the (230Th/238U) data on the effect of the plume. More
Acknowledgements
John Maclennan is thanked for his enthusiasm and help during sample collection in Iceland and for the useful discussions we had during writing up. Julie Prytulak is thanked for providing a thorough review with suggestions that greatly contributed to improvement of the manuscript. Christoph Beier and a third anonymous reviewer are also thanked for their helpful comments. Finally we would like to thank Mark Rehkamper for the editorial handling and helpful additional suggestions and comments. The
References (108)
The generation of uranium series disequilibria by partial melting of spinel peridotite – constraints from partitioning studies
Earth Planet. Sci. Lett.
(1993)- et al.
Ridge-hotspot interaction along the Mid-Atlantic Ridge between 37°30′ and 40°30′N: the U–Th disequilibrium evidence
Earth Planet. Sci. Lett.
(1996) - et al.
U–Th–Pa–Ra systematics for the Grande Comore volcanics: melting processes in an upwelling plume
Earth Planet. Sci. Lett.
(1998) - et al.
Partial melting and upwelling rates beneath the Azores from a U-series isotope perspective
Earth Planet. Sci. Lett.
(2005) - et al.
Melting of enriched mantle beneath Pitcairn seamounts: unusual U–Th–Ra systematics provide insights into melt extraction processes
Earth Planet. Sci. Lett.
(2009) - et al.
Melt migration in plume-ridge systems
Earth Planet. Sci. Lett.
(2003) - et al.
A new Ra–Ba chromatographic separation and its application to Ra mass-spectrometric measurement in volcanic rocks
Chem. Geol.
(1994) - et al.
Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: constraints from U-series disequilibria in lavas from the last quarter-millennium flows
Geochim. Cosmochim. Acta
(2011) - et al.
238U, 234U and 232Th in seawater
Earth Planet. Sci. Lett.
(1986) - et al.
Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction
Earth Planet. Sci. Lett.
(1998)
Melting rates beneath Hawaii: evidence from uranium series isotopes in recent lavas
Earth Planet. Sci. Lett.
230Th–238U disequilibria in historical lavas from Iceland
Earth Planet. Sci. Lett.
Partitioning of U and Th during garnet pyroxenite partial melting: constraints on the source of alkaline ocean island basalts
Earth Planet. Sci. Lett.
Experimental determination of Ra mineral/melt partitioning for feldspars and Ra-226-disequilibrium crystallization ages of plagioclase and alkali-feldspar
Earth Planet. Sci. Lett.
Glacioisostacy controls chemical and isotopic characteristics of tholeiites from the Reykjanes peninsula, SW Iceland
Earth Planet. Sci. Lett.
231Pa and 230Th chronology of mid-ocean ridge basalts
Earth Planet. Sci. Lett.
Experimental and natural partitioning of Th, U, Pb and other trace-elements between garnet, clinopyroxene and basaltic melts
Chem. Geol.
Dynamics of mantle flow and melting at a ridge-centered hotspot: Iceland and the Mid-Atlantic Ridge
Earth Planet. Sci. Lett.
Mantle flow, melting, and dehydration of the Iceland mantle plume
Earth Planet. Sci. Lett.
238U–230Th–226Ra- and 235U–231Pa disequilibria produced by mantle melting with porous and channel flows
Earth Planet. Sci. Lett.
Consequences of diffuse and channelled porous melt migration on uranium series disequilibria
Geochim. Cosmochim. Acta
Experimental constraints on major and trace element partitioning during partial melting of eclogite
Geochim. Cosmochim. Acta
Upwelling and melting of the Iceland plume from radial variation of 238U–230Th disequilibria in postglacial volcanic rocks
Earth Planet. Sci. Lett.
Time-scales for magmatic differentiation at the Snaefellsjokull central volcano, western Iceland: constraints from U–Th–Pa–Ra disequilibria in post-glacial lavas
Geochim. Cosmochim. Acta
A new method for U-Th-Pa-Ra separation and accurate measurement of 234U–230Th–231Pa–226Ra disequilibria in volcanic rocks by MC-ICPMS
Chem. Geol.
Geochemical evolution of historical lavas from Askja Volcano, Iceland: implications for mechanisms and timescales of magmatic differentiation
Geochim. Cosmochim. Acta
Experimental-determination of U-partitioning and Th-partitioning between clinopyroxene and natural and synthetic basaltic liquid
Earth Planet. Sci. Lett.
Investigating solid mantle upwelling rates beneath mid-ocean ridges using U-series disequilibria, 1: a global approach
Earth Planet. Sci. Lett.
A geochemically consistent hypothesis for MORB generation
Chem. Geol.
U-series disequilibria in volcanic rocks from the Canary Islands: plume versus lithospheric melting
Geochim. Cosmochim. Acta
Lead isotope variability in olivine-hosted melt inclusions from Iceland
Geochim. Cosmochim. Acta
Plume-driven upwelling under central Iceland
Earth Planet. Sci. Lett.
Trace element partitioning on the Tinaquillo Lherzolite solidus at 1.5 GPa
Phys. Earth Planet. Inter.
230Th–238U disequilibrium and the melting processes beneath ridge axes
Earth Planet. Sci. Lett.
Source enrichment processes responsible for isotopic anomalies in oceanic island basalts
Geochim. Cosmochim. Acta
238U–230Th constraints on mantle upwelling and plume-ridge interaction along the Reykjanes Ridge
Earth Planet. Sci. Lett.
Observations of 231Pa/235U disequilibrium in volcanic rocks
Earth Planet. Sci. Lett.
TiO2 enrichment in ocean island basalts
Earth Planet. Sci. Lett.
Determining melt productivity of mantle sources from 238U–230Th- and 235U–231Pa disequilibria; an example from Pico Island, Azores
Geochim. Cosmochim. Acta
Assimilation of the plutonic roots of the Andean arc controls variations in U-series disequilibria at Volcan Llaima, Chile
Earth Planet. Sci. Lett.
The dynamics of plume-ridge interaction.1. Ridge-centered plumes
Earth Planet. Sci. Lett.
Mantle melting and magma supply to the Southeast Indian Ridge: the roles of lithology and melting conditions from U-series disequilibria
Earth Planet. Sci. Lett.
Trace element partitioning during the initial stages of melting beneath mid-ocean ridges
Earth Planet. Sci. Lett.
Mantle and crustal contribution in the genesis of recent basalts from off-rift zones in Iceland: constraints from Th, Sr and O isotopes
Earth Planet. Sci. Lett.
Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U–230Th–226Ra and 235U–231Pa disequilibria
Geochim. Cosmochim. Acta
Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise
Geochim. Cosmochim. Acta
Osmium–oxygen isotopic evidence for a recycled and strongly depleted component in the Iceland mantle plume
Earth Planet. Sci. Lett.
Consequences of melt transport for uranium series disequilibrium in young lavas
Earth Planet. Sci. Lett.
The importance of melt extraction for tracing mantle heterogeneity
Geochim. Cosmochim. Acta
Melt extraction in the Earth’s mantle: constraints from U–Th–Pa–Ra studies in oceanic basalts
Earth Planet. Sci. Lett.
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2017, Geochimica et Cosmochimica ActaCitation Excerpt :Mafic mineralogies (pyroxenite, eclogite) have a lower solidus and a higher melt productivity than peridotite (Pertermann and Hirschmann, 2003; Ito and Mahoney, 2005; Prytulak and Elliott, 2009; Bizimis and Peslier, 2015). Koornneef et al. (2012) emphasized the significant effect of lithological heterogeneity on melt productivity in oceanic basalts on the scale of individual eruption centers, based on a relationship between (230Th/238U) and (231Pa/235U) and incompatible trace element ratios in Icelandic basalts. Prytulak and Elliott (2009) calculated melt productivity in Pico using the same method and concluded measured values are consistent with a peridotitic source, albeit independent evidence for a mafic component in the source of Pico lavas (Moreira et al., 1999; see also Millet et al., 2009; Madureira et al., 2011; this study).
- 1
Present address: Institut für Mineralogie, Westfälische Wilhelms Universität, 48149 Münster, Germany.
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Present address: Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon, UCBL and CNRS, Lyon, France.